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Mule is correct about the  problematic thickness of the string but it is really due to the double serving not the strand thickness. Just remove the serving and re-serve yourself (or have a archery shop do it) with a single serving. That worked well for me. see http://tonygt19.smugmug.com/Archery/Dyn … ;k=Mxvx7cC
Cheers,
Tony

For my part I agree strongly with Storm on this point. I push the bow to tilt it forward from the top and also grip the bow almost from the side as the Japanese Yumi archers do. When done correctly with a smooth release the effect is almost magical. The arrow rest is the air. Slightly above your thumb and slightly to the right of the bow (for a right handed thumb ring archer). If done perfectly the twisting torsion on the bow will make the string will align itself to the flight path of the arrow and an arrow of any spine will fly straight out of your bow. Sounds simple but this is the most complex thing in archery. No one can tell you how much to twist or how much to tilt . You have to determine that yourself empirically. It can take years. You have one of only two types of bows that I know of that can be drawn and flexed in 3 dimensions to make the perfect shot. The Japanese Yumi is one and the Korean traditional bow is the other. If you pursue this, once it happens you will be hooked but you may have to shoot a thousand times before it happens again. It is that elusive but your accuracy at short distances will improve remarkably.

From the photo you embedded it looks like you made a beautiful thumb ring but you designed it to function incorrectly. If you are positioning the string between the black ridge and the lighter tab on the ring you will be putting all the draw pressure on the last thumb bone. So the pressure is mostly applied to your thumb print area. That would be extremely painful even with a moderately heavy draw weight. The thumb ring is designed with an oval opening so that it can be slipped over the oval shaped end of the first thumb bone (the one attached to your hand). But when twisted 90 degrees into its functional position it cannot be pulled off without crushing the flange of the bone. That would take well over a hundred pounds of pull even for dainty bones. Nearly all the pressure from drawing the bowstring should be on the 2 red areas which are shown in the photoshopped x-ray I attached. If you pinch those two areas with your other hand and pull as hard as you can you will feel no discomfort. That's more or less the way a correctly fitted thumb ring should feel. This works best when the string is pulled by the upper lip of the ring and slightly above the joint. I pull a 70lb at 28" Hwarang well past 85lbs with no thumb discomfort. Bending your thumb and holding it with your index finger is really only to lock to draw. If your thumb ring can come off at all in its shooting position none of this works. Hope this helps.

Chadwck,
1187shooter's comment is correct. The rigid siyah acts as a lever allowing the archer to pull a short powerful limb into a relatively compact bend which can snap back very quickly compared to a longer true recurve limb which bends along its entire length. For this effect to be maximized the siyah has to be very light but unbending itself.
I might add that siyahs can have more than one stage. I'm attaching a photo of my 70lb  turkish hybrid with the limb sections marked. The bending part of the limb is marked to one side with blue. The siyah is the same length but in two stages marked in green and red. The siyah in Novotny's hybrids is made stiff by inserting and gluing  (very precisely) a stiff curved wedge of maple between the two layers of bamboo. This siyah is designed to act in two stages. The green length is already engaged as a lever when the bow is braced. So it helps you bend  the curved part of the limb (which is as powerful as a crossbow limb) as soon as you begin the draw. The red part of the siyah is not engaged until later in the draw when its part of the bowstring lifts off the flared shoulder (red arrow). Then your lever becomes 50% longer and pulling further becomes  easier and smoother than it would be without this feature.

Selected dried croaker (a type of fish)  air bladders are boiled down into glue. Very tough and flexible. For all the answers to questions like this you should get Bluelake's "Way of the Bow: The Korean Hornbow" DVD. It is the most complete video of hornbow making ever made.

<Ah, I've just seen the 70 lb that you'd posted before, and I have to say, it's a rather beautiful looking bow! How much were you thinking of selling it for? My only concern would be the shipping, as I live in the UK! (And customs- we have some very peculiar customs laws here, regarding weapons and such...)>

Katharine,
I'll be putting the 30 Lb Hwarang up for sale at $370. It sells new for $470. That includes a soft case, silk bow sock and extra string. If you send me your thumb size (width of middle knuckle in mm) I might have a thumbring I can send. I'll have some photos of it up sometime this weekend in the for sale section. You can see it's look alike Hwarangs in 45lb and 70lb on my photosite by googling my forum name and "traditional archery". This bow will challenge you for years. It is very smooth at 30lbs to 28 in but just as smooth up to mid 40 lbs at 32 in (thumbring draw range). Exceptionally lively response to advanced gripping methods but this takes years of practice. See forum member "Storm"'s posts on this. This bow acts more like a true hornbow than my other heavier hwarangs. I think that's due to the limitations of modern materials in flexibility as bow weights progress past 50-70lbs.

Note:
This bow is no longer for sale.

With the heavier draw bow the arrows come out to 6.6 gns/lb. They aren't flying very straight in the first 20 yards so better tuned arrows should go significantly faster.

Your wife is right. This is definitely a luxury item. No one needs a bow that can blow the dust off an elephants ears at fifty yards. We guys just like saying we have one. For my part I'm intrigued with mechanical performance. I've collected bows of ancient design in the 45 and 70 lb range just to test how good they actually are. I'm so annoyed at reading that a bow is "blazing fast" in a test report or that it "launches an arrow with authority". What the hell does that mean? So I got myself a chronograph to make actual measurements. But the Salukis took 18 months to show up. Now we're in the gov "sequester" ( I'm a federal employee) and I ran into some unexpected medical bills. So the ear dusters must go. I can tell you that this bow easily shoots arrows with a 28 " draw past 220 ft /sec.

Yes, during the period of time the string takes to clear the thumb and thumbring (probably a millisecond or less) that long arrow would have the Inertia of a telephone pole. That's why it bends first before actually moving forward. And the string below the nock would suddenly be released more quickly sideways than forward. that would certainly set up oscillations before it became taut.

Are you saying that the inertia of arrow alone  is keeping the top limb from moving until the bottom part of the string becomes taut?If you are right that's a keen insight. Unfortunately the only way to prove these hypotheses is to record these interactions with very high speed video. I see you are already doing that but you may need something close to a 1/50,000 of a sec frame rate to show it clearly. Being a scientist I am very interested in these kinds of investigations. I have some colleagues who do very high speed stop action photos. Maybe through a series of those shots the interaction could be reconstructed. Let me know if you are interested, who knows, perhaps this would be publishable work.